Method for detecting an abnormal disturbance of an internal combustion engine torque

ABSTRACT

A method for detecting an abnormal disturbance of an internal combustion engine torque and for suspending the functioning of a system in which combustion misfiring has been diagnosed. The method operates by analysis of values of a quantity representing the quality of combustion by observing engine crank shaft rotation. A stability criterion is defined of the quantity for each of the engine cylinders. Then, after each combustion the stability criterion is compared with a predetermined threshold, and an abnormal torque disturbance is detected when the stability criterion is greater than the threshold for a given number of consecutive times for at least one cylinder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for detection of an abnormalperturbation of the torque of an internal combustion engine, especiallyone mounted in a motor vehicle. The present invention relates moreparticularly to a process capable of suspending the diagnosis ofmisfires when abnormal perturbations caused in particular by roughroadway pavement or by any other factor create excessive noise in thesignal used for diagnosis of misfires.

2. Discussion of the Background

In the scope of antipollution regulations such as the American OBDstandards (On Board Diagnostics), it is required that electronic enginecontrol systems of motor vehicles must be able to diagnose certainengine dysfunctions which influence pollutant emissions.

For this reason it is planned, for future electronic engine controlsystems, to install systems capable of diagnosing proper operation ofthe oxygen sensor or of the EGR loop (exhaust gas recirculation),wherein recognition of a malfunction which affects pollutant emissionsmust trip activation of less intensive modes of operation and/or mustturn on a light on the dashboard to warn the driver (relaxedregulations) or must stop the vehicle (strict regulations).

In particular, the American and European standards provide for requiringdetection of misfires and for identification of the cylinder orcylinders responsible. Such detection must, for example, furnish thepercentage of misfires occurring in a given number of engine cycles.

The equipment designers and automobile manufacturers have thereforedeveloped a certain number of techniques for detection of misfires.

As an example, there can be cited the accelerometer method, whichcomprises detecting a misfire by analysis of the variation oflongitudinal acceleration of the vehicle, or the method using anoxygen-proportional sensor disposed in the exhaust line. There can alsobe cited the method using pressure sensors communicating with thecombustion chambers, or that based on measurement of arc voltage orionization current of the spark plugs (for controlled ignition engines).

The most commonly used method, however, is that which deduces theexistence of misfires by measurement of the instantaneous speed of thecrankshaft. The use thereof is actually extremely simple, since itinvolves merely applying software processing of the signal furnished bythe angular position sensor of the crankshaft, a signal which is alreadyused by the engine control system to control fuel injection, and so nospecific equipment-related device is required for implementation of thismethod.

The analysis solely of the signal furnished by the crankshaft positionsensor to detect possible misfires nevertheless suffers from certaindisadvantages.

The method of detection of misfires is in fact based on the postulatethat a misfire is manifested by a drop of the gas torque, which in turngenerates a corresponding change of the instantaneous speed of thecrankshaft. To identify misfires, therefore, it is sufficient to recordthe changes in instantaneous speed of the crankshaft.

However, the instantaneous speed of revolution of the crankshaft and ofthe engine flywheel integral therewith reflects not only the operationof the engine and the alternating thrust of the connecting rods underthe effect of combustion of the carbureted mixture, but also theoperation of the entire kinematic chain connecting the engine to thetire/ground interface.

In fact, the mechanical energy at the end of the crankshaft istransmitted to the wheels by a transmission system which traditionallycomprises a clutch, a speed-change box and a differential, thistransmission possessing its own damping and stiffness. As a result,therefore, any abrupt variation of torque involving any of the elementsof the kinematic chain, such as, for example, involving the vehicle'swheels because of poor condition of the roadway surface, is fed back tothe crankshaft in the form of oscillations, the magnitude of which willdepend on the characteristics of the transmission system and of theperturbation.

As a result, the abrupt changes in instantaneous speed of the crankshaftare therefore caused not solely by misfires but also by all theperturbations capable of affecting the kinematic transmission chain, andtherefore especially a roadway in poor condition.

For correct and exclusive identification of misfires, therefore, itseems important to be able to distinguish, among the changes ofcrankshaft speed, those due effectively to drops of gas torque fromthose having other causes, so that the latter are not counted.

To achieve this, additional strategies for deactivation of the programfor detection of misfires have been developed, which strategies arebased on recognition of perturbations affecting the kinematictransmission chain. Thus there can be cited strategies which use theinformation of wheel speed furnished by a specific sensor or else thoserequiring an accelerometer. There can also be cited the method describedin British Patent GB A 2290870, which attempts to identify suchperturbations by fuel motion in the fuel tank.

It nevertheless appears at present that none of the proposed methods iscapable of distinguishing, in simple and economic fashion and withsufficient precision and reliability, the changes of crankshaft speedcaused by perturbations which affect the kinematic transmission chain.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to improve the processdescribed in the foregoing by providing a process for detection ofabnormal torque perturbations other than those related to true misfires,which process is capable of temporarily suspending the diagnosis ofmisfires.

The process for detection of an abnormal perturbation of an internalcombustion engine torque and suspending the operation of a system fordiagnosis of misfires is of the type that operates by analysis of thevalues of a variable representative of firing quality by observation ofthe rotation of the engine crankshaft.

According to the invention, the process for detection of an abnormaltorque perturbation is characterized in that it comprises:

defining a criterion of stability of the variable representative offiring quality for each cylinder of the engine and for each firing;

comparing, after each firing, this stability criterion with apredetermined threshold, and detecting an abnormal torque perturbationwhen this stability criterion exceeds, for at least one cylinder, thethreshold for a given number of consecutive times.

According to another characteristic of the process according to theinvention for detecting an abnormal torque perturbation, the saidvariable representative of firing quality is the gas torque.

According to another characteristic of the process according to theinvention for detecting an abnormal torque perturbation, the stabilitycriterion quantifies, for a given cylinder and given firing, theabsolute deviation between the value of the said variable representativeof firing quality and the value of a statistical variable representativeof the mean of the different values of the variable representative offiring quality, the values being measured successively over a givenmeasurement horizon.

According to another characteristic of the process according to theinvention for detecting an abnormal torque perturbation, the statisticalvariable representative of the mean of the different values of thevariable representative of firing quality is obtained by a first-orderlow-pass filter with a given filtering constant.

According to another characteristic of the process according to theinvention for detecting an abnormal torque perturbation, the thresholdis deduced from the variations of the variable representative of firingquality observed during normal engine operation, and it thereforedepends on the engine operating point.

According to another characteristic of the process according to theinvention for detecting an abnormal torque perturbation, the firingsbeing diagnosed as misfires initiate a particular treatment in thecalculation of the stability criterion.

According to another characteristic of the process according to theinvention for detecting an abnormal torque perturbation, the firingsbeing diagnosed as misfires initiate a particular treatment in thecomparison of the stability criterion with the predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives, aspects and advantages of the present invention will bebetter understood from the description presented hereinafter ofdifferent embodiments of the invention, given as non-limitativeexamples, with reference to the attached drawings, wherein:

FIG. 1 is a partial view of an internal combustion engine equipped withan engine control system permitting use of the process according to theinvention;

FIGS. 2a to 2 c are timing diagrams indicating the principle of theprocess according to the invention for detection of abnormal torqueperturbations;

FIG. 3 is a flow chart indicating the sequence of the process accordingto the invention for detection of abnormal torque perturbations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there can be seen a simplified sketch of an enginecontrol system using the process according to the present invention fordetecting misfires. Only the constituent parts necessary forunderstanding of the invention have been shown.

Internal combustion engine 1 is designed more particularly as equipmentfor a motor or road vehicle. Engine 1 is connected to a transmissiondevice suitable for transmitting motion to the vehicle's wheels. Thistransmission device classically comprises a clutch, a speed-change box 2and a differential, not illustrated.

The four-cycle multi-cylinder engine 1 is equipped with a fuel-injectiondevice of the multi-point type with electronic control, by virtue ofwhich each cylinder is supplied with fuel from a specific electronicinjector 5.

Opening of each electronic injector 5 is controlled by the electronicengine control system 7, which adjusts the quantity of fuel injected andthe moment of injection into the cycle as a function of engine operatingconditions, in such a way that the richness of the combustible air-fuelmixture admitted to the cylinders is adjusted automatically andprecisely to a predetermined setpoint value.

The electronic engine control system 7 classically comprises amicroprocessor 70, read-write memories 71, read-only memories 72,analog-to-digital converters 74 and various input and output interfaces.

Microprocessor 70 is provided with electronic circuits and softwareroutines appropriate for processing the signals originating fromsuitable sensors, for determining the states of the engine and forinitiating predefined operations in order to generate control signalsdestined in particular for the injectors (and for the ignition coils inthe case of a controlled ignition engine), so as to ensure optimummanagement of the firing conditions in the engine cylinders.

The input signals of microprocessor 70 include in particular thoseaddressed by a crankshaft sensor 4. This sensor 4, of the variablereluctance type, for example, is mounted immovably on the engine frameat a position in front of a measuring gear 12 fixed to one end of thecrankshaft.

This gear 12 is provided on its periphery with a succession of teeth andspaces, which are identical with the exception of one tooth, which hasbeen removed in order to define an absolute reference with which therecan be deduced the moment at which a given reference cylinder, in thepresent case cylinder No. 1, passes top dead center.

Sensor 4 delivers a signal Dn corresponding to the procession of theteeth of gear 12, which signal is processed to generate a TDC signalduring each half-revolution of the crankshaft, thus making it possibleto identify in alternation the passes of cylinders No. 1, 3, 4, 2through top dead center.

Processing of signal Dn emitted by sensor 4 also makes it possible tomeasure the speed of procession of the teeth of gear 12, and thus torecord the instantaneous speed of revolution of the engine.

Microprocessor 70 therefore transforms signal Dn to produce arepresentative variable characteristic of the quality of firingsoccurring in each of the engine cylinders and to perform the diagnosisof misfires according to a process known in itself, which will not bedescribed in detail since it is not the object of the present invention.

The representative variable characteristic of firing quality can be, forexample, the instantaneous crankshaft acceleration (see German PatentApplications DE 3939113 or DE 4002208), or else the torque, the value ofwhich is obtained by spectral analysis of the instantaneous crankshaftspeed (see French Patent Applications No. 91/11273 and 91/11274 filed bythe Applicant in relation to a “process and device for measuring the gastorque of an internal combustion heat engine”).

According to the embodiment described below, the gas torque Cg is therepresentative variable chosen to control both the system for diagnosisof misfires and the system capable of suspending the diagnosis ofmisfires when perturbations external to engine operation are interferingwith this diagnosis.

During operation of the engine, appropriate calculating means activatedby microprocessor 70 therefore furnish successive torque values Cg,n,ito characterize the value of the gas torque Cg corresponding to the n-thfiring of cylinder No. i.

These values are then processed by first calculation means designed toperform the diagnosis of misfires and by second calculation means,specific to the present invention, designed to identify the occurrenceof torque perturbations capable of falsifying the diagnosis of misfires,which perturbations are due, for example, to the transmission device andin particular to the condition of the roadway pavement on which thevehicle is traveling.

According to the invention, the process for detecting the occurrence oftorque perturbations capable of falsifying the diagnosis of misfires isperformed by analysis of the torque variations cylinder by cylinder. Inthe practical example described hereinafter, it is sufficient to detectthe occurrence of torque perturbations in a single cylinder in order tosuspend the diagnosis of misfires for all cylinders.

Quite obviously it can be provided, in alternative embodiments, that theoccurrence of torque perturbations in a single cylinder suspends thediagnosis of misfires only for the cylinder in question, or else it canbe provided that the occurrence of torque perturbations must be detectedin a predetermined number of cylinders in order to suspend the diagnosisof combustion misfires for all cylinders.

Similarly, in the event of detection of abnormal torque perturbations,inhibition of the diagnosis of misfires can be tripped immediatelythereafter or can even be effected retroactively on a given periodpreceding the detection of the perturbations, in order to allow for theresponse time, the length of which depends on the chosen embodiment ofthe process for detection of perturbations.

In the process according to the invention, therefore, the first step iscalculation of a statistical variable MCg,n,i representative of the meanof the different torque values measured successively on cylinder No. iover a given horizon of torque measurements.

Calculation of the statistical variable MCg,n,i is achieved, forexample, by a first-order low-pass filter with given filtering constantτ. In this case, statistical value MCg,n,i for any new torque valueCg,n,i is determined from the previously calculated statistical value,MCg,n-1,i, on the basis of the following recurrence formula:

MCg,n,i=MCg,n−1,i+((Cg,n,i−(MCg,n−1,i))/τ)

The value thus calculated from the statistical variable MCg,n,irepresentative of the mean of the different torque values Cg,n,imeasured successively for cylinder No. 1 over a given horizon of torquemeasurements is then used to determine the value ECg,n,i, defined as theabsolute deviation between the torque value Cg,n,i and the statisticalvalue MCg,n,i:

ECg,n,i=|MCg,n,i−cg,n,i|

This deviation ECg,n,i is therefore representative of the engine torquevariations for the cylinder in question and therefore of the stabilityof this torque.

In an alternative embodiment, ECg,n,i can also be obtained, not from thevalue MCg,n,i, but instead from the value MCg,n−1,i calculated in thepreceding cycle, specifically in order to calculate ECg,n,i morerapidly. This yields:

 ECg,n,i=|Mcg,n−1,i−Cg,n,i|

In an alternative embodiment of the process according to the invention,it is also possible to eliminate from the calculation of MCg,n,i theeffect of misfires which occurred during the observed period and whichwere detected by the said first calculation means. For this purpose, thetorque value corresponding to a misfire is replaced in the calculationof the statistical variable MCg,n,i by the last value that did notcorrespond to a misfire or else, by blocking the filter, to itspreceding value MCg,n,i=MCg,n−1,i.

Quite obviously, if the cylinder continuously exhibits misfires(ignition defect, etc.), detection of abnormal perturbations is thensuspended for that cylinder until the fault is cleared.

Regardless of the formula for calculating ECg,n,i, it is thensufficient, in order precisely to isolate the abnormal torqueperturbations (rough roadway pavement, transmission jolts, etc.) capableof interfering with the diagnosis of misfires, to compare this deviationwith a predetermined fixed stability threshold Sstab, which depends onthe engine operating point.

The stability threshold Sstab is deduced from the maximum torqueexcursions encountered during normal operation. It therefore depends onthe engine operating point and is obtained, for example, from thefollowing formula:

 Sstab=α·δCg

According to this formula, α is a calibrated factor (0<α<1) and δCg isthe expected torque drop at the operating point. Sstab, which is listedin tables or obtained by calculation, can also evolve with aging of theengine. Of course, any other representation of the nominal torquedelivered by the engine can be used instead of the torque drop.

The graphs of FIGS. 2a, 2 b and 2 c illustrate the principle of theinvention.

For stabilized engine operating conditions, when the transmission deviceis not subject to any large perturbation, and in the absence of misfire(FIG. 2a), the values of the gas torque Cg,n,i for a given cylinder ithen vary only slightly on both sides of the value MCg,n,i. Thecriterion ECg,n,i then remains close to 0 and therefore is always belowthe threshold Sstab.

When misfires occur (FIG. 2b), the criterion ECg,n,i reaches extremevalues during each misfire, because the instantaneous torque Cg,n,i thendrops abruptly while the value MCg,n,i remains at a high value, whereasfor the other firings the values of gas torque Cg,n,i vary only slightlyon both sides of the value MCg,n,i and the criterion ECg,n,i thenremains close to 0. Thus the threshold Sstab is exceeded onlysporadically, and only during the occurrence of misfires.

In contrast, during perturbations external to cylinder operation, as inthe case of poor roadway condition (see FIG. 2c), the torque Cg,n,i isthen randomly noisy, as is the stability criterion ECg,n,i, whichreaches a much higher mean level. The threshold Sstab is then frequentlyexceeded.

It is therefore sufficient to note the frequency at which the thresholdSstab is exceeded in order to detect the occurrence of abnormal torqueperturbations. For example, it is sufficient, in order to deduce theoccurrence of abnormal torque perturbations, to record p consecutiveovershoots of the threshold Sstab by the criterion ECg,n,i, where p is anatural integer greater than or equal to 2, and to suspend the diagnosisof misfires.

Nevertheless, in order not to confuse the occurrence of abnormal torqueperturbations with a cylinder dysfunction which continuously generatesmisfires, overshoots of the threshold Sstab can be counted only in thecase of a misfire.

As illustrated in FIG. 3, a process according to the invention thereforecomprises the following stages, which are executed after each firingindexed n and for a given cylinder No. i:

(i) acquisition of the value of the gas torque Cg,n,i generated byfiring, and calculation of the value ECg,n,i (using MCg,n−1,i);

(ii) comparison of ECg,n,i with Sstab

if ECg,n,i is less than Sstab, counter N is reset to 0 (N=0);

if ECg,n,i is greater than or equal to Sstab, counter N is incrementedby one unit (N=N+1);

(iii) at the same time, MCg,n,i is calculated;

(iv) comparison of N with p

if the value of counter N is less than p, it is therefore assumed thatabnormal torque perturbations have not been detected

if, on the other hand, the value of counter N is greater than or equalto p, it is then assumed that abnormal torque perturbations have beendetected, and the diagnosis of misfires is then suspended.

The described process for suspending the diagnosis of misfires thereforehas the advantage of being a particularly simple and rapid method ofachieving detection of abnormal torque perturbations capable offalsifying the diagnosis of misfires, without extra cost due to acomponent. Furthermore, this process is remarkably reliable andtherefore trips inhibition of the diagnosis of misfires only whennecessary.

Of course, the invention is in no way limited to the described andillustrated embodiment, which has been presented only as an example.

To the contrary, the invention comprises all technical equivalents ofthe described means as well as combinations thereof if they are effectedin accordance with its spirit.

Thus, it is possible to operate not directly on the values Cg,n,i but onfiltered values.

As regards implementation of the device for detection of misfires, itcan be achieved in diverse forms regardless of the variant chosen:

with analog electronic components, in which case the summing units,comparators and other filters are achieved by means of operationalamplifiers;

or with digital electronic components, which would achieve the functionby hard-wired logic;

or by a signal-processing algorithm loaded in the form of a softwaremodule, which is a component of an engine control software system foroperating the microcontroller of an electronic calculator,

or even by a specific (custom) chip, whose hardware and softwareresources will have been optimized to achieve the functions according tothe invention; chip microprogrammable or not, encapsulated separately orelse comprising all or part of a coprocessor implanted in amicrocontroller or microprocessor, etc.

Similarly, the invention comprises all technical equivalents applied toan internal combustion engine, regardless of its combustion cycle(2-cycle, 4-cycle), of the fuel used (diesel or gasoline), or even ofthe number of its cylinders.

What is claimed is:
 1. A process for detecting an abnormal perturbationof a torque of an internal combustion engine and suspending an operationof a system for diagnosis of misfires, which system operates by analysisof values of a variable representative of firing quality by observationof rotation of an engine crankshaft, said process comprising the stepsof: defining a criterion of stability of said variable for each cylinderof the engine and for each firing; comparing, after each firing, thestability criterion with a predetermined threshold; and detecting anabnormal torque perturbation when said stability criterion exceeds, forat least one cylinder, the threshold for a given number of consecutivetimes, wherein said stability criterion quantifies, for a given cylinderand given firing, an absolute deviation between the value of saidvariable and the value of a statistical variable representative of amean of different values of said variable.
 2. A process for detecting anabnormal torque perturbation according to claim 1, wherein said variablerepresentative of firing quality is a gas torque.
 3. A process fordetecting an abnormal torque perturbation according to claim 1, whereinthe value of said variable and the value of said statistical variableare measured successively over a given measurement horizon.
 4. A processfor detecting an abnormal torque perturbation according to claim 3,wherein said statistical variable representative of the means of thedifferent values of said variable is obtained by a first-order low-passfilter with a given filtering constant.
 5. A process for detecting anabnormal torque perturbation according to claim 1, wherein saidpredetermined threshold is deduced from variations of a statisticalvariable observed during normal engine operation, and therefore dependson an engine operating point.
 6. A process for detecting an abnormaltorque perturbation according to claim 1, wherein the firings diagnosedas misfires initiate a particular treatment in the calculation of saidstability criterion.
 7. A process for detecting an abnormal torqueperturbation according to claim 1, wherein the firings diagnosed asmisfires initiate a particular treatment in the step of comparing thestability criterion with said predetermined threshold.
 8. A process fordetecting an abnormal perturbation of a torque of an internal combustionengine and suspending an operation of a system for diagnosis ofmisfires, which system operates by analysis of values of a variablerepresentative of firing quality by observation of rotation of an enginecrankshaft, said process comprising the steps of: defining a criterionof stability of said variable for each cylinder of the engine and foreach firing; comparing, after each firing, the stability criterion witha predetermined threshold; and detecting an abnormal torque perturbationwhen said stability criterion exceeds, for at least one cylinder, thethreshold for a given number of consecutive times, wherein saidpredetermined threshold is deduced from variations of a statisticalvariable observed during normal engine operation, and therefore dependson an engine operating point.
 9. A process for detecting an abnormalperturbation of a torque of an internal combustion engine and suspendingan operation of a system for diagnosis of misfires, which systemoperates by analysis of values of a variable representative of firingquality by observation of rotation of an engine crankshaft, said processcomprising the steps of: defining a criterion of stability of saidvariable for each cylinder of the engine and for each firing; comparing,after each firing, the stability criterion with a predeterminedthreshold; and detecting an abnormal torque perturbation when saidstability criterion exceeds, for at least one cylinder, the thresholdfor a given number of consecutive times, wherein the firings beingdiagnosed as misfires initiate a particular treatment in the calculationof said stability criterion.
 10. A process for detecting an abnormalperturbation of a torque of an internal combustion engine and suspendingan operation of a system for diagnosis of misfires, which systemoperates by analysis of values of a variable representative of firingquality by observation of rotation of an engine crankshaft, said processcomprising the steps of: defining a criterion of stability of saidvariable for each cylinder of the engine and for each firing; comparing,after each firing, the stability criterion with a predeterminedthreshold; and detecting an abnormal torque perturbation when saidstability criterion exceeds, for at least one cylinder, the thresholdfor a given number of consecutive times, wherein the firings beingdiagnosed as misfires initiate a particular treatment in the step ofcomparing the stability criterion with said predetermined threshold.